An ionic liquid (also called as an ion liquid) is a salt composed of a cation and anion, which liquid is a compound having a melting point approximately equal to or lower than an ambient temperature, and has variety of characteristics such as flame resistance, non-volatility, high thermal stability, and so on. As an example of such ionic liquid, there has been known a quaternary ammonium salt disclosed in Patent Literature 1. The quaternary ammonium salt includes one(1) to four alkoxyalkyl groups as substituent groups thereof, and it is described that such ionic liquid is used as an antibiotic, a cation active cleaning agent, an emulsifying agent for protection and corrosion prevention of architectural structures and for refining of ores, or dyeing auxiliary.
In addition, since the ionic liquid has good flame resistance and thermal stability, it can be used also as an electrolyte solvent of an electrochemical device, for example, as an electrolyte solvent for a lithium ion secondary battery. Thus, by using the ionic liquid as the electrolyte solvent, improvement of electrochemical stability, namely, improvement of stability as a battery can be expected.
As examples of actually using the ionic liquid as the electrolyte solvent, there have been known Patent Literatures 2 to 4, which are relevant to ionic liquids of quaternary ammonium salts.
In this regard, however, though the claims of Patent Literature 2 describe including a quaternary ammonium salt in which one(1) to four substituent groups thereof are alkoxyalkyl groups, there are only examples, obtained by conducting actual experiments, where the quaternary ammonium salt includes only one(1) alkoxyalkyl group as substituent group thereof. Thus, the cases that two or more substituent groups are alkoxyalkyl groups have not been confirmed. Additionally, an example of a cathode active material for the lithium ion secondary battery, which has been confirmed in the examples, is only lithium cobalt oxide. Although lithium cobalt oxide is widely used as the cathode active material for the lithium ion secondary battery, it is expensive because cobalt is a rare metal. Moreover, there is a problem that lithium cobalt oxide has poor thermal stability and is likely to degrade and release oxygen when temperature becomes high. For these reasons, there have been needs for the cathode active material which is inexpensive and has good thermal stability.
Recently, as the cathode active material which meets the above needs in stead of lithium cobalt oxide, lithium oxo acid salt (oxo acid salts of lithium) such as lithium iron phosphate and lithium iron silicate has attracted attention. Lithium oxo acid salt is composed of relatively inexpensive elements, and has good thermal stability. Accordingly, lithium oxo acid salt is expected to be used as the cathode active material for the secondary battery, as the material which is inexpensive, has high level of safety, and can obtain high capacity depending on the composition thereof.